Big success in the film business often means opening a can of worms along with the champagne. The Palme d'Or at this year's Cannes film festival went to the epic and erotic love story Blue Is the Warmest Colour. But the jury and its president, Steven Spielberg, insisted the prize should be accepted not only by the director, Franco-Tunisian film-maker Abdellatif Kechiche, but also by his two young stars, Léa Seydoux and Adèle Exarchopoulos.

Julie Maroh, who wrote the original graphic novel, dismissed Kechiche's adaptation as a straight person's fantasy of gay love. As for Kechiche, his feelings about that last-minute requirement to share the Palme with his two actors can only be guessed at – and the same goes for their feelings about his feelings. Seydoux and Exarchopoulos have since said he was oppressive, intrusive, and even tyrannical in the demands he made, especially in the extended explicit sex scene, which took fully 10 days to shoot.

Led by this internal dissent, the film's critical tide may be slowing, if not turning. But I think that the impact of the movie increases with a second viewing, and my own objections about the lovers' ferocious "confrontation" scene have been answered. It no longer looks melodramatic, but rather the icy and violent culmination of a hitherto invisible disconnect between the two women. This drama was never supposed to celebrate the equality of their romantic good faith. Its original French title is perhaps a better guide: La Vie d'Adèle Chapitres 1 et 2. Adèle, played by Exarchopoulos, is the sympathetic centre of the story, a schoolgirl at the beginning and a teacher by the end: the two chapters of innocence and experience.

What a passionate film it is. At the outset, Exarchopoulos's Adèle is a shy, smart high-schooler who finds that she is lonely and tentative in her social life. A good-looking boy who likes her is rewarded with a brief relationship, but he is merely John the Baptist to the imminent Christ: Emma, played by Séydoux, a twentysomething art student. The romantic spark between them is a lightning bolt.

As for the much discussed sex scene, I predicted earlier this year that some sophisticates would claim to find it "boring". The second charge, that it is exploitative or inauthentic, is also naive. It is no more authentic or inauthentic than any sex scene, or washing-up scene, or checking-in-at-the-airport scene. It is fictional. The sequence certainly strikes me as uncompromising and less exploitative than any smug softcore romcom or mainstream thriller in which women's implied sexual availability is casually served up as part of the entertainment, although I will concede one tiny moment of misjudgment: when Emma is painting a nude of Adèle (unfortunately like Leo and Kate in Titanic) and the camera travels up her naked body.

When the love affair starts, Emma has blue hair; as it proceeds, the blue colour grows out. As Kechiche shows, that is a bad sign. Their love is cooling. Emma is always the senior, dominant partner: better educated, more worldly and higher up the social scale. Kechiche sketches this out by having Emma bring Adèle around for dinner with her mum and stepdad. There is no secret about their relationship, and they stylishly have oysters. When Emma meets Adèle's conservative folks, however, the food is humbler – spag bol – and Emma has to pretend to have a boyfriend. And when Emma's art career takes off, Kechiche shows how she is starting inexorably to outgrow Adèle, and yet it is Adèle who develops a kind of emotional maturity that Emma, the increasingly smug careerist, can't match.

The movie's final sequence is heart-stoppingly ambiguous. Yet the point is surely that there is no guarantee that either Adèle or Emma will ever find anything as good ever again. The notion that they can each go on to find a better or richer experience is illusory. This isn't young love or first love, it is love: as cataclysmic and destructive and sensual and unforgettable as the real thing must always be. To paraphrase Woody Allen, if it doesn't make the rest of your life look like a massive letdown then you're not doing it right. Here is Emma and Adèle's moment, the definitive blaze.

Blue Is the Warmest Colour really is an outstanding film and the performances from Exarchopoulos and Séydoux make other people's acting look very weak.